1. Field of the Invention
The present invention relates to the design of pressurized plastic containers useful for storing carbonated beverages.
A container for carbonated beverages generally consists of a finish portion with a handling ledge which blends into a neck portion of the container. The neck portion blends into the main body of the container which may assume various configurations. Presently known containers are relatively high in weight which adds cost to their production. Similarly, presently available containers suffer from nonuniform orientation of the plastic in the main body portion of the container. Such nonuniform orientation produces lower barrier properties and lower thermal stability than needed for many applications which require long shelf life and high mechanical strength of the overall container.
2. Description of the References
In the past, many methods for fabricating organic thermoplastic, oriented containers have been used. Similarly, many designs for containers are known in the art.
For example, U.S. Pat. No. 4,153,667 to Brady et al. discloses a method for enhancing the yield strength and density of an oriented plastic container wherein the material is susceptible to strain-hardening. The plastic material is thermally preconditioned to a temperature within the molecular orientation range and is then mechanically conditioned by stretching in a first direction to a length between 2.6-2.7 times its original length. Subsequently, the plastic material is stretched in a second direction, with strain-hardening and strain-induced crystallization thereby increasing drastically, substantially immediately, upon the initiation of the second stretching operation due to the mechanical conditioning by elongation during the first stretching operation.
Another method for conditioning strain-hardenable thermoplastic materials for fabricating containers is disclosed in U.S. Pat. No. 4,144,298 to Lee. The Lee patent discloses a method of conditioning strain-hardenable thermoplastic materials so that a highly developed strain-crystallized morphology is established during the blow molding operation. In this method a thermoplastic parison is heated to a temperature in the range conductive to molecular orientation and then initially stretched at that temperature. Next, the stretched parison is cooled to a temperature slightly below the glass transition temperature of the plastic and stretched further at the reduced temperature. The combined stretching at the two separate phases conditions the material such that it is either on the verge of being strain-hardened or is actually strain-hardened before any further processing.
Another approach to container manufacture is disclosed in the patent to Farrell, U.S. Pat. No. 3,972,976. The Farrell patent discloses a method for the longitudinal stretching of a parison to be blow molded. The Farrell method includes a special provision for maintaining the plastic of the parison on a blow molding machine's core rod at the orientation temperature of the plastic, and stretching the plastic of the parison lengthwise of the core rod. The lengthwise stretching of the parison in the direction of one axis without substantial increase in the diameter of the parison occurs. After this orientation in one axis has occurred, the temperature is controlled to maintain an orientation temperature. Next, the plastic of the parison is blown to a larger diameter so as to obtain orientation in the second direction. The resultant container is biaxially oriented.
Another approach to container manufacture is disclosed in the patent Agrawal et al., U.S. Pat. No. 4,131,666, which discloses a method for forming molecularly oriented containers from reheated parisons. The method includes heating the parisons to orientation temperature followed by distention to container form in a closed mold. The resultant container includes improved reduced thickness variability in lower portions of the container which result from controlling preformed shrinkage in the heating step of between 4-15 percent of the initial length of the material. This is achieved by maintaining the ratio of average thickness to inside parison surface area within defined limits. The preforms are then axially and radially stretched during the distention to the predetermined level.
Among the objectives of the present invention are the provision of a novel container structure for a container fabricated from orientable, strain-hardenable, organic, thermoplastic materials, which includes a number of advantageous features. The container according to the present invention is extremely lightweight compared to commercially available containers used for holding similar volumes. Further, the container according to the present invention has highly oriented, very uniform sidewalls which provide superior barrier properties and high thermal stability yielding long shelf life and high mechanical durability. Further, the present container provides for a limited concave blending portion between the neck of the container and the shoulder portion of the main body of the container.
More specifically, the container according to the present invention provides for highly efficient material distribution to achieve a uniform, highly oriented sidewall in the main body portion of the container. Also, limited concave surface area on the container is achieved. The minimization of concave surface area is advantageous to lower such surface area which is subject to very high stress due to constant forces exerted upon such concave surface areas by internal carbonation pressure. Upon long exposure to such internal pressure the highly stressed concave positions of the container develop stress cracking. Such stress cracking lowers the mechanically stability of the neck portion of the container.
Additionally, the container according to the present invention exhibits high stretch ratios in the main body portions of the container to achieve significant high levels of orientation with the above described advantages. Further, the container according to the present invention includes minimal surface area for a predefined volume to most efficiently approximate a spherical configuration. The minimal surface to volume ratio produces a superior container with lower gas transmission rates, and thus higher shelf life than commercially available containers.